The present invention relates to an ultrasonic testing apparatus and method for rapidly inspecting a large number of gas cylinders of similar design for defects located in the internal neck-shoulder region. More particularly, the apparatus and method of the present invention allows for the rapid production of ultrasonic images of the internal neck-shoulder region of the gas cylinders in order to uncover radially oriented defects such as cracks or folds. Such defects, if undetected, can result in leakage of the contents of the gas cylinder. This may be particularly dangerous in case a toxic substance is stored in the gas cylinder.
In the prior art, the neck-shoulder region of a gas cylinder is inspected by removing the valve from the head of the gas cylinder and then inserting a dental mirror into the head opening of the gas cylinder. A light is then directed down the head opening and the mirror is maneuvered for visual inspection of the internal neck-shoulder region of the gas cylinder for defects. A major problem with this technique is that it is extremely time consuming and therefore, not practical if large numbers of cylinder are to be inspected. Moreover, cracks growing from folds may be easily overlooked. In some cases, small harmless folds may exist in the shoulder, but it is often difficult to judge how deep they may be and these cylinders may be unnecessarily rejected. Thus, neck-shoulder region defects in gas cylinders are either missed; or when found, can result in a unnecessary withdrawal of the gas cylinder from service.
The prior art has provided ultrasonic testing techniques to inspect welds and surfaces, such as the inner and outer surfaces of a pipe or a plate, for cracks, voids and other defects. In an ultrasonic testing technique referred to as the angle-beam technique, an ultrasonic transducer is mounted within a transducer shoe at an oblique vertical angle, called a wedge angle, to insure that the ultrasonic pulse emitted by the transducer skips between the opposed surfaces of the pipe or plate at points called nodes and thereby encounters the defects at an angle to produce return echoes. It should be noted that return echoes are produced at gas-metal interfaces, for instance, at the air gap located between the internal surfaces of a crack. If a pulse encounters a crack in a direction parallel to the plane of the crack, there is simply not enough area of interface to produce a return echo. The skip distance, that is the distance between two nodes, is determined by calculation, measurement by a separate transducer, or by an angle beam test block, in order to thereby determine the area in which the transducer is to be moved to scan an area of interest. Typically, the area of interest is scanned by moving the shoe in a zig-zag path, parallel to the area of interest, between one-half and one full skip distance from the area of interest. Additionally, the shoe may be positioned at a fixed point, typically one full skip distance or less from the area of interest and horizontally swiveled or skewed from side to side to determine the depth of the defect.
The above-mentioned angle-beam technique may be performed by a skilled technician manually positioning the shoe. A processor is provided to activate the transducer to emit ultrasonic pulses and to respond to return echoes, produced by the reflection of the ultrasonic pulses, by graphically displaying the amplitude of the return echoes against time on an oscilloscope. The skilled technician then interprets the graphical display on the scope to determine the size and the position of defects. The prior art has also provided automated testing apparatus that automatically scans the article being tested and automatically processes the ultrasonic pulses and return echoes to produce interpretive displays.
The automated scanning apparatus of the prior art moves the shoe and thus, the transducer through a predetermined path to scan the area of interest. Such scanning apparatus may include a wheeled vehicle having magnetic wheels that ride on a steel, band-like track that is attached to the article being tested; for instance, the circumference of a pipe. The shoe housing the transducer is connected to the wheeled vehicle by means of an arm that can retract and extend the shoe in directions toward and away from the wheeled vehicle to scan an area of interest as the vehicle travels around the track.
The interpretive display produced by such automated prior art testing apparatus may utilize three display modes to graphically display the defects, namely, an A-scan, a B-scan and a C-scan. The A-scan displays a plot of return echo signal amplitude versus time. Defect size may be estimated by comparing the return echo signal amplitude with that produced by a flaw of known size and shape. The B-scan displays a plot of time versus the position of the transducer. Return echo signal amplitude is indicated by preprogrammed brightness or color indications on the plot. Such plot is used to indicate the position and the orientation of flaws and defects in the article being tested. Lastly, the C-scan display is a plot of a plan or view of the article being tested with flaws and defects superimposed over such plan view. Automated processing systems that display C-scan plots have electronic depth gate circuitry to limit the number of echo signals that are processed within a preselected range of signal delay times. This allows a region of the article, parallel to the surface being scanned, to be selected for display.
Angle beam ultrasonic testing techniques, either manual or automatic, have not heretofor been utilized in the testing of the neck-shoulder region of gas cylinder heads for cracks and folds because the head of the gas cylinder is not longitudinally symmetrical. Here it is relevant to point out in the performance of any ultrasonic inspection technique, the shoe must be positioned flush against a sound conducting couplant coating the outer surface of the article to be tested. If a gap exists between the couplant and the shoe, the ultrasonic pulse will not enter the article. If one were to conventionally test the internal neck-shoulder region of the cylinder head for defects by the angle beam technique, the change of surface slope at the juncture of the head and shoulder of the cylinder would make the requisite contact between the shoe and the outer surface impossible. Moreover, since angle-beam techniques contemplate moving the shoe in a predetermined pattern in a region near the defect, such change and surface slope would prevent the required movement of the shoe. It should be mentioned that one skilled in the art would not be led to scan the material neck-shoulder region of the gas cylinder from the shoulder region because the longitudinal asymmetry of the gas cylinder head causes the skip distance to vary with the thickness and change in surface of the cylinder head; and therefore, there has been no known method to compute the wedge angle. Another reason as to why one would not be led to position the shoe on the shoulder region of the gas cylinder is that radially oriented defects would be scanned edge wise, parallel to the extent of the defects, and therefore not produce a return echo.
If the above-mentioned problems associated with ultrasonically scanning the internal neck-shoulder region of a gas cylinder were solved, a large number of gas cylinders could be rapidly inspected for neck-shoulder defects. However, a skilled technician would be required to perform the test and interpret the results. As will be discussed, automated ultrasonic testing apparatus is used in the present invention to even more rapidly inspect large numbers of gas cylinders for neck-shoulder defects; and further to eliminate the need for a skilled technician to perform the inspection. However, such automated ultrasonic testing apparatus must be set up on each similar gas cylinder in a prescribed location and orientation. The "Set-up" of such automated apparatus is time consuming in and of itself and, thus such apparatus does not readily lend itself to the inspection of a large number of gas cylinder for neck-shoulder defects.
As will be described in more detail hereinbelow the present invention provides an ultrasonic testing apparatus and method for testing the neck-shoulder region of a gas cylinder for defects. The method and apparatus of the present invention lends itself for use with automated scanning apparatus with elimination of set-up time delay so that the ultrasonic test may be performed on a large number of gas cylinders without any special exercise of skill or training required on the part of the person performing the test.